Labour needs to occupy the Opposition benches with its taste for power intact, as it failed to do after Thatcher's victory in 1979

So this is how it ends. As the outgoing Prime Minister kicked his heels in Downing Street waiting for the Clegg-Cameron negotiations to end, his ally-turned-enemy-turned-ally again Lord Mandelson went to Buckingham Palace ahead of him to chair his last meeting of the Privy Council as Lord President, to secure the Royal signature on such momentous legal instruments as an amendment to the charter of Goldsmiths College and the Red Meat Industry (Wales) Measure. With such Merrie England panoply the 13-year-old era of New Labour drew towards its stately close, last night's gracious resignation statement by Gordon Brown, and his own last visit to the Palace as Prime Minister.

Now Labour has to relearn the habits – once all too ingrained – of opposition. And to do so after a surreal 24 hours of national uncertainty as Britain seemed to swing wildly between the prospect of being a social democratic country or, as it has turned out, an unprecedented hybrid of conservatism laced – we still don't quite know how strongly – with liberalism. And it did so with a kind of melancholy relief that the agony was over, on a day when several of the Labour equivalents of grandees queued up to assert that the time had come to leave the field and acknowledge defeat.

The stance of the Jack Straws, John Reids and David Blunketts is hardly new. No currently active politician knows more about what it is like to go from government to opposition in a hung parliament than Kenneth Clarke, who was a whip in Edward Heath's government when it left office in February 1974.During the 2010 campaign he reminisced, prophetically as it now seems, about how it fell to him to sound out Tory parliamentarians as Heath – like Mr Brown 36 years later in second place – was desperately trying to bring Jeremy Thorpe's Liberals on board. One "sensible old boy", the backbencher Kenneth Lewis, had told him bluntly: "You tell that prime minister of ours that he has just lost the election and he should go with dignity."

To his credit, Mr Brown, by his clear statement on Monday, had removed any lingering sense that Heath-like he was remaining in No 10 for the sake of preserving his own premiership. Of course there were arguments for letting go; to attack the deficit will be a formidable enough task without having to assemble a fragile overall parliamentary majority each night. But either way the tantalising and subversive moment when it seemed that the electoral arithmetic could be used to deliver a left-of-centre government passed almost as quickly as it had arrived.

Which does not mean that Labour can relax. For that same arithmetic means that Labour needs to occupy the Opposition benches with its taste for power intact, as it signally failed to do after Margaret Thatcher's victory in 1979. It is not only that she had an overall majority and David Cameron's Conservatives do not; it is also that the combination of searing ideological division and rebarbative policies on defence and the economy, from which the party took more than a decade to recover, is mostly no more than distant memory.

Labour, in other words, has a chance of winning the next whenever it comes, under a new leader. And it needs to be ready when the moment arrives. Nor is it easy to see, when a general election takes only four weeks, why a leadership contest should take as long as three months.

Which means in turn that the conduct as well as the content of the leadership election is of paramount importance. First, it will need to bear the stamp of a genuinely new era. The big figures, the ruling quadrangle as it seemed for much of the last 13 years – Gordon Brown, Tony Blair, Peter Mandelson and Alastair Campbell – would surely do well to keep out of it, signalling by their absence from the fray that a new generation is taking over. And secondly, that the habits of gang warfare – too prevalent for much of the last 13 years – will surely need to be banished.

It's hard to believe that the party will forgive revival of the dark arts of denigratory briefing, often through a right-wing press which will gratefully lap it up, during a contest of incalculable importance to Labour's future. For each one of the candidates will have to remember that the party is on show to the electorate; it does not have the luxury, as it did for much of the Eighties, of waging civil war secure in the knowledge that it was not a serious alternative government.

By making it clear last night that he was resigning forthwith as Labour leader, Mr Brown signalled that it will begin immediately. For much of the day he had waited in Downing Street, not the squatter he had been depicted as, but by now more like a prisoner awaiting his release. It came just after 7.15pm. The era that began on 1 May 1997 was over.

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